1. Origin of the Moon
2 September 2015

2. Why study the origin of the moon?
How terrestrial planets form: they build up from impacts between smaller objects
Moon effects on Earth: tides, change Earth spin
Pluto’s moons likely formed the same way, from a giant impact on Pluto

3. Why study the origin of the moon?
Effects of Moon on Earth:
Tides
Obliquity stabilized
Day and month changes

4. terrestrial planets formation
Disk of gas and dust around Sun
Interparticle collisions: if impact velocities are low enough, we get gravitationally bound aggregates
10,000 yrs: 10 km-sized bodies
100,000 yrs: Moon-Mars sized (~2000 km, ~20 “embryos”)
1 million-10 million yrs: planet-sized “giant impacts” will reduce number of embryos to 4 terrestrial planets
Should produce ~20 terrestraial planetary “embroys” on circular orbits

5. Evidence for giant impacts
Planets spin faster than they orbit
Planets are tilted to orbital revolution
Should produce ~20 terrestrial planetary “embroys” on circular orbits

6. Moon Properties
Name some of the distinguishing properties of the Moon…
Lunar density vs. earth density
Lunar similarity to earth’s mantle
Angular momentum
Expansion of lunar orbit
Circular orbit

7. Moon Properties Earth has only 1 Moon Depleted in Fe and volatiles;
Oxygen isotopes similar to Earth
Moon’s orbit:
is not in Earth’s equatorial plane
Circular
Expanding due to tidal interaction
Moon has very small core
Lunar density vs. earth density
Lunar similarity to earth’s mantle
Angular momentum
Expansion of lunar orbit
VOLATILE: evaporates at lower temps (water, N, etc)
Circular orbit

8. Moon Origin Hypotheses
Co-accretion: Earth and Moon formed together. Like sister
Fission: Earth spun so fast that it split off a Moon-sized chunk. Like daughter
Capture: Earth captured an independently-formed Moon as it passed by. Like wife. THESE WERE THE 3 HYPOTHESES BEFORE APOLLO!
Giant Impact: Mars-sized body collided with proto-Earth and excavated material eventually coalesced to form Moon

9. Evaluate the Hypotheses
Co-accretion: Moon has little iron, volatiles. Fission: Earth never spun fast enough Capture: too unlikely AFTER APOLLO WE STILL HAD THE SAME THREE POSSIBILITIES
Flip back to “moon properties” slide to help them fill in the table

10. Giant Impact Stages Earth close to final size both differentiated
Mars-sized impactor
both differentiated
both formed near 1 AU

11. Where does Iron go?

12. Where does Iron go? Both Fe cores stay with Earth
1 lunar mass in orbit outside Roche radius
Moon is mostly impactor material
Where does Iron go?

13. heat removes volatiles from debris disk
How hot is the Impact?

14. Evolution of the Protolunar disk
Centrally condensed hot disk <a> = REarth
Cooling: condensation/solidification
Collisional spreading of disk
Accretional growth of moonlets
Tidal evolution of moonlets
Collisions between moonlets yield moon

15. the post-impact moon Impact: Mars-sized body collides with Earth
Debris ejected into Earth orbit
A. heated
B. comes from mantle (no Fe)
C. ~1 lunar mass = ~1% Earth mass = ~10% impactor mass
Debris accumulates to form one large Moon, not multiple small moons… but maybe a second, smaller moon hits it later

16. ReAccretion & the post-impact moon
Earth spin and Moon orbit locked
Moon orbit expands a few cm/yr
Earth rotation slows: conservation of angular momentum

17. ReAccretion & the post-impact moon
In the past, which is a possible state of the Earth/Moon system?
A. Moon orbits closer in, Earth’s day is 18 hours
B. Moon orbits farther away, Earth’s day is 36 hours
C. Moon orbits closer in, Earth day is same as now
D. Same conditions as today A

18. ReAccretion & the post-impact moon
In the past, which is a possible state of the Earth/Moon system?
A. Moon orbits closer in, Earth’s day is 18 hours
B. Moon orbits farther away, Earth’s day is 36 hours
C. Moon orbits closer in, Earth day is same as now
D. Same conditions as today